1. Field of the Invention
The present invention relates to a focal-plane shutter apparatus provided in an image-pickup apparatus such as a single-lens reflex digital camera.
2. Description of the Related Art
Focal-plane shutter apparatuses include leading blade that are driven for exposure from a state of shutting a shutter aperture (hereinafter referred to as a “shut state”) to a state of opening the shutter aperture (hereinafter referred to as an “opened state”) and trailing blades that are driven for the exposure from the opened state to the shut state. The leading blades and the trailing blades are respectively constituted by plural light-blocking blades. The plural light-blocking blades are connected to each other by an arm member so as to form a parallel link, and thereby the light-blocking blades are parallel moved.
The arm member is driven by a driving lever that is urged by a spring. The driving lever is provided with an armature that is attracted and held by an energized electromagnet at a position where the spring is charged (hereinafter referred to as a “charged position”).
Cut-off of the energization to the electromagnet allows an urging force of the spring to rotate the driving lever, and thereby the driving lever drives the plural light-blocking blades through the arm member. When exposure thus performed by movement of the leading blades and the trailing blades, a charging mechanism including a motor returns the driving levers for the leading and trailing blades to the respective charged positions while charging the spring, and returns the leading and trailing blades to respective movement start positions.
Japanese Patent Laid-Open No. 11-194394 discloses a focal-plane shutter apparatus that includes a cam member on which two cam surfaces for leading blades and trailing blades are formed which respectively return a leading blade driving lever and a trailing blade driving lever to charged positions where springs are charged, and a charging mechanism having a motor that rotates the cam member. The charging mechanism rotates the motor in one direction to cause the cam surfaces to retract from the driving levers that have been attracted by electromagnets via an armature at the charged positions, and then cuts off energization of the electromagnets to allow rotation of the driving levers by urging forces of the springs. Further, the charging mechanism drives the driving levers by the cam surfaces to the charged positions after exposure. Each of the two cam surfaces for the leading and trailing blades is formed to have an inclination portion gradually retracting from the driving lever such that no impact is generated at an attracted part of the electromagnet and the armature due to the urging force of the spring when the cam surface retracts from the driving lever located at the charged position. Angular areas where the inclination portions for the leading and trailing blade driving levers are formed on the cam member are equal to each other.
Moreover, some single-lens reflex digital cameras provided with such a shutter apparatus include a so-called live-view display function that opens the shutter apparatus before image capturing and then displays a moving image generated from outputs from an image-pickup element on a monitor.
When the live-view display is performed, the energization to the electromagnet for the leading blades is cut off, and thereby the leading blades are moved to the opened state. On the other hand, although it is necessary to also maintain the trailing blades in the opened state, continuous energization to the electromagnet for the trailing blades increases power consumption. Therefore, the shutter apparatus disclosed in Japanese Patent Laid-Open No. 11-194394 holds the trailing blade driving lever at the charged position by the cam surface for the trailing blades formed on the cam member, thereby eliminating necessity of energizing the electromagnet for the trailing blades.
However, since the angular areas where the inclination portions for the leading and trailing blade driving levers are formed on the cam member are equal to each other and a rotation speed of the motor is slow, a time required for returning the leading blade driving lever to the charged position becomes long. Therefore, a release time lag from a release instruction to exposure becomes long in a normal image capturing.